There are actually a few good tutorials out there on this forum for evocation in general. Here are a few of them:
There are also many books out there dealing with invoking angels and demons, such as Gallery of Magic. As for gods, a simple evocation tends to work with communicating with them, although there are cultural methods that seem to help make the connection. Here are some books I have read that have helped with different Pantheons:
Celtic:
Irish Paganism by Morgan Daimler
The Apple Branch: A path to Celtic Ritual by Alexei Kondratiev
Norse:
Essential Asatru by Diana L. Paxson
Gods and Myths of Northern Europe by Hilda Ellis Davidson (this book is not a ritual book but goes in depth with the history behind the stories that may help you get ideas of what to include in ritual to help establish the connection)
A Practical Heathen’s Guide to Asatru by Patricia Lafaylive
Hellenic
Hellenic Polytheism: Household Worship by Christos Pandion Panopoulos
Hellenismos: Practicing Greek Polytheism today by Tony Mierzwicki
Hope this helps, good luck!